Based on the requirements of light source used in fluorescence polarized immunoassay analysis device, light emitting diode (LED) is proposed to use as stimulating source and illuminating system with small size, compact structure and high collimation is designed. Referring to the luminous characteristics of LED, a new system combined with refractive and reflective freeform surfaces is designed for collimating. Equations between the source and target surfaces are presented through the theory of geometrical optics to get the surface parameters. The simulation results show that the collimating system can achieve ± 2.4° diverging angel for a 1 mm × 1 mm LED surface source with efficiency no less than 85%. Accomplishing the expected of compact construction and high collimation, the paper offers an effective reference for compact design of fluorescence polarized immunoassay analysis device.